Pregnant female rats injected with Methylazomethanol-acetate (MAM) on the 15th day of gestation produce offspring with marked micrencephaly. The greatest reduction in brain size is seen in the cerebral hemispheres which are reduced to about 60 per cent of normal size: the structures most affected in the cerebral hemispheres are the posterior neocortex and limbic cortex. The micrencephalic rats survive and reproduce in complex social/physical environments, but exhibit a reduced capacity to perform appropriate social and parental behaviors in two main respects: (1) they become involved in many more wound-resulting aggressive encounters and (2) they desert and cannibalize their young at a much higher rate than do controls. These differences in behavior occur predominately during the initial phases of exposure to the complex situations, and, because later on the animals are somewhat better able to cope with these situations, it is suggested that the brain-damaged individuals adapt more slowly to new, stressful circumstances. Evidence based on brain weight and histological examination of the brains suggests that, not only the reduction in size, but, also the associated disorganization of the cytoarchitecture of the cerebral cortex are important factors contributing to the abnormal behavior.